1. Field of the Invention
The present invention relates to a method and system for optical fiber transmission using Raman amplification.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) silica optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying an optical signal or signal light has been put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light.
For example, an erbium doped fiber amplifier (EDFA) has already been developed to amplify signal light in a 1.55 μm band where the loss in a silica fiber is low. The EDFA includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 μm band or a 1.48 μm band, a gain band including a wavelength of 1.55 μm can be obtained.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. At a receiving end, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
By using an optical amplifier as a linear repeater, the number of parts in the repeater can be greatly reduced as compared with the case of using a conventional regenerative repeater, thereby ensuring reliability and allowing a substantial cost reduction.
Recently, there has been extensively examined the application of an optical repeater using Raman amplification capable of further reducing noise and broadening the band in addition to or in place of an EDFA. In the Raman amplification, an optical fiber generally used as an optical fiber transmission line is used as an optical amplifying medium, and pump light is supplied to the optical fiber. As a pumping source used in the Raman amplification, a high-power pumping source is required. Accordingly, it is expected that a recent tendency of a laser diode (LD) to have a high power and a high efficiency can accelerate practical utilization of the optical repeater using Raman amplification. Further, also in a remote amplifying method such that pumping is performed from an end of an optical fiber transmission line without using an optical repeater, the Raman amplification using a general optical fiber as an optical amplifying medium is useful in providing a distributed amplification system.
In a Raman amplification process using a single pumping source, an obtainable gain band (a band where gain can be provided) is relatively narrow. Accordingly, a plurality of pumping sources for outputting pump lights having different wavelengths are practically used to thereby extend the gain band. However, since the Raman amplification process utilizes conversion of the power of pump light into the power of signal light, the gain that can be obtained varies according to the power of signal light included in the gain band.
For example, in the case of using Raman amplification for repeating of WDM signal light, the total power changes with a change in number of wavelength channels of the WDM signal light, causing a change in the gain that can be obtained. Specifically, in the case that two pumping sources are used, the gain given to optical signals having wavelengths included in a gain band obtained by one of the two pumping sources becomes different from the gain given to optical signals having wavelengths included in a gain band obtained by the other pumping source. As a result, there is a possibility that the transmission characteristics of optical signals obtaining a high gain may be degraded by nonlinear phenomena.
It has been proposed to prevent such a degradation in transmission characteristics by transmitting a supervisory signal to a Raman repeater by means of signal light to adjust an operational balance among a plurality of pumping sources. In this case, however, there is a limit to the number of signals transmittable because signal light is used for the transmission of the supervisory signal.